For capsulation by covering a core in a fine particle form with a polymer, so-called microcapsulation, a number of methods are known, including the phase-separation method, the interfacial polymerization method, the in situ polymerization method and the spray drying method.
In microcapsulation by the in situ polymerization method among the above methods, generally by carrying out polymerization in a dispersion system in which a core material and reactants, i.e., reaction components such as a monomer and a prepolymer necessary to form a polymer, or a catalyst coexist, a polymer insoluble in the core material and a dispersion medium is formed in the interface therebetween and thus a shell of the polymer is formed on the surface of particles of the core material, whereby capsulation is realized.
However, in microcapsulation by the conventional in situ polymerization method, in practice, capsulation is rarely achieved as described above; it has disadvantages in that the polymer formed by polymerization does not sufficiently undergo phase-separation from the core particle and is entrained in the core particle, thereby failing to form a shell, or even if the polymer is formed in the condition that it has undergone phase-separation from the core particle, the shell is formed insufficiently and the core in the inside is exposed, and thus capsulation is incomplete. Thus, in fact, the capsulation by the in situ polymerization method has not almost been put into industrial use although it has advantages in that the process is simple and the size of the capsule or the thickness of the shell is easy to control.
Heretofore, as methods for the production of hollow capsule-shaped polymer particles (hereinafter referred to as "hollow polymer particles"), for example, the following have been known:
(I) a method in which a foaming agent is incorporated in polymer particles and afterwards the foaming agent is expanded;
(II) a method in which a volatile substance such as butane is incorporated in a polymer and afterwards it is gassified and expanded;
(III) a method in which a polymer is melted, and gas such as air is jetted into the molten polymer to thereby incorporate air bubbles in the polymer; and
(IV) a method in which an alkali swelling substance is incorporated in the inside of polymer particles, and an alkaline liquid is permeated through the polymer particles to swell the alkali swelling substance.
However, these methods are all difficult in controlling conditions and it has been difficult to produce the desired hollow polymer particles in high yield and with high reliability.